1. Field of the Invention
This invention relates to microemulsions suitable for use in many applications such as fragrances, cleaning fluids, paint vehicles, toxicants and antiseptics, cosmetics and toiletries, and foods and flavors and pharmaceuticals.
2. Description of the Prior Art
A microemulsion is an extraordinary kind of emulsion that forms spontaneously. Products consisting of these systems are prized for their stability and small particle size, which gives them special consideration in the marketplace.
Microemulsions are stable dispersions of one liquid in another in the form of droplets, the diameter of which are less than one-quarter the wave length of white light. Although this size requirement means that light can pass through it, the system is not necessarily transparent. It is generally referred to as translucent which term is defined to include "transparency," which may be described as excellent translucency. Such systems remain dispersed and will not as macroemulsions do, achieve equilibrium by separating into the original, mutually insoluble liquid phases. While with the aid of surfactants, it is possible to produce macroemulsions which remain dispersed for periods of time as long as 24 hours or possibly more, ultimately they achieve equilibrium by separating into the original, mutually insoluble liquid phases. On the other hand, microemulsions exhibit stability which may be measured in years. Historically such systems have been referred to as "solubilized" rather than considered to be emulsions.
Microemulsions are recognized by the physical properties displayed by their small droplets. The optical properties of these micro droplets and their behavior in a gravitational field easily differentiate them from macro droplets.
Microemulsions are particularly useful and in some cases essential as compared to macroemulsions for many applications. For example, it is often desirable to emulsify water-insoluble fragrances for mouthwashes, shaving lotions, or similar products in order to prevent them from precipitating out of the alcohol-water mixtures in cold weather. Microemulsification in transparent form is essential to maintain the "polished" clarity of these products. Also, water-in-oil emulsions of cleaning fluids make it possible for dirt and stains to be exposed to both liquids during the cleaning operation. Microemulsions increase the efficiency of the mixture and prolong its life. Possibly the largest single use of microemulsions is in the paint industry. Many paint vehicles are oil-in-water microemulsions of alkyds or polymers. The stability of these vehicles insures uniformity of application and coherent films. The small particle size of the microemulsions enables high gloss finishes to be easily formulated. The use of water instead of smelly and noxious solvents has enormous consumer appeal and in industrial use saves money on fire insurance premiums and reduces air pollution.
Many products in the cosmetic and toiletries area, such as cold creams, shampoos, etc., are microemulsions. Usually, they are used to insure stablity or transparency, but frequently advantage is taken of the light scattering properties (translucency or opalescence) of micro droplets to appeal to the consumer. Microemulsions of water-insoluble materials are utilized in the food industry primarily to meet stringent stability requirements. Any breakdown of the emulsion during storage or shipment with consequent deterioration of appearance could be disastrous. For this reason, transparent systems with large excesses of edible emulsifiers are usually employed. Microemulsions of flavor oils in cola, cream soda, and other confections are typical examples.
Many drugs are insoluble in water, but it is desirable for one reason or another to administer them in an aqueous medium. Where accurate dosage is required so that uniformity of dispersion is essential, microemulsification is indicated. In the case of consumer items for oral ingestion, (e.g., vitamin oils), the transparency of a stable microemulsion has sales appeal.
While there is a strong need for microemulsions, as is evident from the previous discussion, unfortunately producing such emulsions is often difficult, if not impossible. It is generally necessary to test a wide variety of surfactants to try to achieve such microemulsion in a wide variety of chemical types as stated in Emulsions and Emulsion Technology, part 1, by Kenneth J. Lissant, published by Marcel Dekker, Inc., New York, 1974, page 167, which says, "Although this method of finding a microemulsifier is a workable one, it is obviously time-consuming and its chances of success without a great many experiments are small." (Emphasis added.)